Position detection device and a position detection method for a workpiece to be welded

Abstract

A position detection device for a workpiece to be welded including a spot welding gun having a pair of electrodes adapted to be disposed opposite each other across the workpiece; a robot for holding either one of the spot welding gun and the workpiece in a manner movable relate to each other; a servo motor for allowing the pair of electrodes to approach the workpiece and separate from the workpiece; a physical quantity detection section for detecting a physical quantity correlative to a torque of the servo motor when the servo motor allows one of the pair of electrodes to approach a surface of the workpiece so that the one of the pair of electrodes abuts against the surface of the workpiece; a position detection section for detecting positions of the pair of electrodes; a storage section for storing the physical quantity detected by the physical quantity detection section and a value detected by the position detection section; and a computation section for calculating a contact start time at which the one of the pair of electrodes comes into contact with the surface of the workpiece based on time-series data of the physical quantity stored in the storage section, and computing a position of the workpiece at the contact start time based on the value detected by the position detection section stored in the storage section.

1 . A position detection device for a workpiece to be welded comprising:
 a spot welding gun having a pair of electrodes adapted to be disposed opposite each other across the workpiece;   a robot for holding either one of the spot welding gun and the workpiece in a manner movable relative to each other;   a servo motor for allowing the pair of electrodes to approach the workpiece and separate from the workpiece;   a physical quantity detection section for detecting a physical quantity correlative to a torque of the servo motor when the servo motor allows one of the pair of electrodes to approach a surface of the workpiece so that the one of the pair of electrodes abuts against the surface of the workpiece;   a position detection section for detecting positions of the pair of electrodes;   a storage section for storing the physical quantity detected by the physical quantity detection section and a value detected by the position detection section; and   a computation section for calculating a contact start time at which the one of the pair of electrodes comes into contact with the surface of the workpiece based on time-series data of the physical quantity stored in the storage section, and computing a position of the workpiece at the contact start time based on the value detected by the position detection section stored in the storage section.   
     
     
         2 . A position detection device for a workpiece to be welded according to  claim 1 ,
 further comprising a control section for controlling the servo motor,   wherein the computation section has a determination section for determining whether the one of the pair of electrodes is in a predetermined pushing state after the one of the pair of electrodes makes contact with the surface of the workpiece or not, based on the physical quantity detected by the physical quantity detection section, and   wherein the control section controls the servo motor to stop approaching the one of the pair of electrodes toward the surface of the workpiece when the determination section determines that the one of the pair of electrodes is in the predetermined pushing state.   
     
     
         3 . A position detection device for a workpiece to be welded according to  claim 2 , wherein the determination section determines that the one of the pair of electrodes is in the predetermined pushing state when the physical quantity detected by the physical quantity detection section becomes equal to or larger than a predetermined value, or when an increasing rate of the physical quantity detected by the physical quantity detection section per unit time becomes equal to or larger than a predetermined value. 
     
     
         4 . A position detection device for a workpiece to be welded according to  claim 2 , wherein the determination section determines that the one of the pair of electrodes is in the predetermined pushing state when the physical quantity detected by the physical quantity detection section becomes larger than a physical quantity in a reference state by a predetermined amount or more, the reference state being a state in which the physical quantity is substantially constant before the one of the electrodes makes contact with the surface of the workpiece, or when an increasing rate of the physical quantity detected by the physical quantity detection section per unit time becomes larger than an increasing rate of the physical quantity per unit time in the reference state by a predetermined amount or more. 
     
     
         5 . A position detection device for a workpiece to be welded according to  claim 4 , wherein the reference state is a state in which the one of the pair of electrodes approaches the surface of the workpiece at a constant velocity. 
     
     
         6 . A position detection device for a workpiece to be welded according to  claim 2 , wherein the computation section calculates, as the contact start time, a second time at which the physical quantity stored in the storage section becomes less than the physical quantity at a first time by a predetermined amount or more, going back from the first time at which the determination section determines that the one of the pair of electrodes is in the predetermined pushing state. 
     
     
         7 . A position detection device for a workpiece to be welded according to  claim 2 , wherein the computation section calculates, as the contact start time, a second time at which an increasing rate of the physical quantity stored in the storage section per unit time becomes 0 or a negative value, going back from a first time at which the determination section determines that the one of the pair of electrodes is in the predetermined pushing state. 
     
     
         8 . A position detection device for a workpiece to be welded according to  claim 2 , wherein the control section controls the servo motor based on a predetermined working program for carrying out spot welding. 
     
     
         9 . A position detection device for a workpiece to be welded according to  claim 2 , wherein the computation section calculates a position detection correction amount of the one of the pair of electrodes, based on the value detected by the position detection section at the contact start time, stored in the storage section, and the value detected by the position detection section when the control section stops the one of the pair of electrodes, stored in the storage section, and computes the position of the workpiece based on the position detection correction amount and the value detected by the position detection section when the one of the pair of electrodes is stopped. 
     
     
         10 . A position detection method for a workpiece to be welded for detecting a position of the workpiece includes the steps of:
 holding, by a robot, either one of a spot welding gun and the workpiece in a manner movable relative to each other, the spot welding gun having a pair of electrodes adapted to be disposed oppositely to each other across the workpiece;   allowing, by a servo motor, one of the pair of electrodes to approach a surface of the workpiece so that the one of the pair of electrodes abuts against the surface of the workpiece;   judging a contact start time at which the one of the pair of electrodes comes into contact with the surface of the workpiece, based on a physical quantity correlative to a torque of the servo motor when the one of the pair of electrodes approaches the surface of the workpiece; and   computing a position of the workpiece based on positions of the pair of electrodes at the contact start time.